The present invention generally relates to synthesizers and voltage tunable dielectric capacitors.
Prior art synthesizers, especially those intended for the 5–13 GHz frequency range, can be classified into five broad categories: synthesizers incorporating VCO's with GaAs-based varactors; synthesizers incorporating VCO's with Si-based varactors; synthesizers incorporating YIG-tuned oscillators; synthesizers incorporating VCO's with active elements other than silicon bipolar transistors; and synthesizers incorporating VCO's with frequency multiplied outputs. However all of the above have disadvantages and shortcomings.
The disadvantage of synthesizers incorporating VCO's with GaAs-based varactors is higher levels of phase noise due to the lower Q factor and higher noise generated by GaAs-based varactors. A significant disadvantage to synthesizers incorporating VCO's with Si-based varactors is higher levels of phase noise due to the lower Q factor and higher noise generated by Si-based varactors. Major distadvantages of synthesizers incorporating YIG-tuned oscillators are the oscillator has a larger physical volume and mass, it has higher power consumption and slower switching speed due to the heavy magnetic circuit. YIG-tuned oscillators are also prone to microphonics and phase hits.
A drawback of synthesizers incorporating VCO's with active elements other than silicon bipolar transistors is higher levels of phase noise due to the higher flicker-noise corner frequency of other active devices. Lastly, disadvantages of synthesizers incorporating VCO's with frequency multiplied outputs are higher levels of far-out phase noise and sub-harmonics due to frequency multiplication process
The terms Parascan® voltage tunable capacitors, Parascan® variable capacitors, Parascan® tunable dielectric capacitors and Parascan® varactors have the same meaning and are interchangeable. Common varactors used today are Silicon and GaAs based diodes. The performance of these varactors is defined by the capacitance ratio, Cmax/Cmin, frequency range and figure of merit, or Q factor (1/tan) at the specified frequency range. The Q factors for these semiconductor varactors for frequencies up to 2 GHz are usually very good. However, at frequencies above 2 GHz, the Q factors of these varactors degrade rapidly. At 10 GHz the Q factors for these varactors are usually only about 30.
Varactors that utilize a thin film ferroelectric ceramic as a voltage tunable element in combination with a superconducting element have been described. For example, U.S. Pat. No. 5,640,042 discloses a thin film ferroelectric varactor having a carrier substrate layer, a high temperature superconducting layer deposited on the substrate, a thin film dielectric deposited on the metallic layer, and a plurality of metallic conductive means disposed on the thin film dielectric, which are placed in electrical contact with RF transmission lines in tuning devices. Another tunable capacitor using a ferroelectric element in combination with a superconducting element is disclosed in U.S. Pat. No. 5,721,194.
Commonly owned U.S. patent application Ser. No. 09/419,126, filed Oct. 15, 1999, and titled “Voltage Tunable Varactors And Tunable Devices Including Such Varactors”, discloses voltage tunable varactors that operate at room temperature and various devices that include such varactors. Commonly owned U.S. patent application Ser. No. 09/434,433, filed Nov. 4, 1999, and titled “Ferroelectric Varactor With Built-In DC Blocks” discloses voltage tunable varactors that include built-in DC blocking capacitors. These varactors operate at room temperatures to provide a tunable capacitance.
The assignee of the present invention and in the patent and patent applications incorporated by reference has developed the materials technology that enables these tunable varactors which have high Q values resulting low losses and extremely high IP3 characteristics, even at high frequencies. The articulation of the novel tunable material technology is elaborated on in the patent and patent application incorporated in by reference.
Therefore, a strong need in the synthesizer industry exists for synthesizers incorporating voltage controlled oscillators (VCO's) that can utilize novel voltage tunable dielectric capacitors to overcome the aforementioned disadvantages of existing synthesizers.